MHC-based reagents play an important role in the study of virus specific CD4 and CDS T cells. These reagents enable detection, characterization and purification of epitope-specific T cells by flow cytometry. With this application, research project Investigators propose studies that examine the roles of virus-specific CD4 and CDS T cells in immunopathogenesis of category A-C viral infections, and in vaccine induced immunity. The MHC Reagents Core will produce both MHC-I and II reagents for these studies. We will utilize established techniques for the expression and purification of MHC I and II molecules and will maintain inventories of these proteins in storage for generating peptide-MHC complexes as needed by Investigators for their studies. As peptide-MHC complexes are generated, they too will be placed in repository. These peptide-MHC complexes will largely be used to generate fluorochrome-labeled tetramers for studies involving flow cytometry. Analyses using novel techniques employing these reagents are being developed by the Technology Development Project. All complexed and multimerized reagents will undergo rigorous quality control testing using established column chromatography techniques and western blotting to detect properly folded molecules, and using flow cytometry to determine if cell lines with known specificities (provided by the Clinical Research Core) are appropriately labeled. As novel MHC I or II reagents are developed by the Technology Development Project, this Core will take over production at a late stage, when expression and re-folding protocols have been established. This Core will serve an important role for virtually all projects in this Program.